Thousands of people have been forced to flee from escalating violence in towns in eastern Congo after renewed fighting between rebels and pro-government militia.

Despite a declared ceasefire by the Tutsi rebel commander, Laurent Nkunda, on 29 October, Nyanzale and Kikuku in the North Kivu province of the country have experienced fighting after an outbreak around the town of Kiwanja, 45 miles (72km) north of the regional capital, Goma, on Tuesday.

A United Nations military spokesman said that the rebels were seeking to extend the territory in their control in the province, despite the ceasefire.

Lieutenant-Colonel Jean-Paul Dietrich, from the UN peacekeeping force, said: "[The rebels] have taken Nyanzale and Kikuku, therefore breaking their own declared ceasefire. Now it's clear they are trying to have a territory completely under their control."

He said the UN and human rights experts were looking into reports that Tutsi fighters had killed civilians when they drove pro-government Mai Mai militia from Kiwanja.

A Belgian journalist working for a German newspaper is also believed to have been kidnapped by the Mai Mai – one of the dozens of militias involved in the fighting – in a village near Kiwanja on Tuesday along with an assistant and three rebel fighters. A local official said the government was negotiating their release.

Violence erupted after Nkunda accused the Congolese army of firing mortars at rebel positions from behind Mai Mai lines around Kiwanja, and of breaking the ceasefire he declared after forcing government troops to retreat to the outskirts of Goma. Nkunda also accused Rwandan Hutu militias linked to the 1994 genocide of fighting alongside the Mai Mai around Kiwanja.

Nkunda said the latest round of fighting in Nyanzale was caused by pro-government militia attacks. "We were attacked three times this morning. My soldiers have a right to defend themselves," Nkunda said. He said there had also been fighting in two other towns in the region, Mweso and Kashuga.

A wider ceasefire between the rebels and the government appeared to be holding around Goma.

Mothers and babies are among those forced to find safety after continued fighting in and around. Hundreds of others were forced to take shelter in a roofless, abandoned school near a United Nations base manned by Indian peacekeepers, but later joined a stream of refugees leaving the area after receiving no help.

"[The UN] should open up their gates to protect us," said Ntaganzwi Sinzahera, who initially took refuge at the school.

Few had time to gather any possessions as rebel loyalists searched houses. A few residents remained, peeking out of homes on to deserted streets.

"Tonight we don't know where we're going. I didn't bring anything. We don't have any food," said Omar Issa, one of the crowd leaving the town.

The violence comes as the UN secretary general, Ban Ki-moon, prepared to meet Joseph Kabila, the Congolese president, and Paul Kagame, president of Rwanda, at a regional peace summit in Kenya on Friday.

Rebels have accused regional powers of becoming involved in the fighting, including Angola and Zimbabwe, while the government believes Rwanda has been aiding the militias.

Fighting in the Congo has intensified since August and Nkunda has warned of resuming war against the Congolese government if he is not involved in direct talks.

Although Kagame is believed to have strong influence over Nkunda's rebel army, the government has yet to sit down with rebel and militia groups.

Ban said he would encourage the leaders at the summit in Nairobi to find a path to peace. "I will certainly encourage [Kabila] to engage in dialogue with whoever, including Nkunda. I will discuss this matter with [him]," he said.

Aid agencies said they were struggling to provide food and medical care for more than 200,000 refugees crammed into camps around Goma. They said there could be another million people who were still out of reach of help, either cut off by the fighting or hiding in the bush.

A spokeswoman for Médecins Sans Frontières, one of the organisations helping with the relief effort, said they were extremely concerned for people displaced by the fighting in eastern Congo. "Significantly greater and more widespread aid is urgently needed," she said.